The present invention relates to supercharger seals and in particular to equalizing a pressure difference across a supercharger rotor shaft seal.
Power production of an internal combustion engine is ultimately limited by the amount of air pumped through each engine cylinder. Fuel systems can at best provide an optimal amount of fuel to burn with the air contained in the cylinder, and adding more fuel than required for a stoichiometric air-fuel ratio does not result in more energy being produced. The power production of non-supercharged engines is thus limited by the engine's ability to draw air into each cylinder, referred to the Volumetric Efficiency (VE) of the engine, where 100 percent VE is equivalent to complete filling of the cylinder at bottom dead center at one atmosphere of pressure. While some engines achieve greater than 100 percent VE using tuned intake manifolds providing a ram effect, the effects are generally limited to a small RPM range which the intake is tuned to.
Power production may also be realized by raising the RPM that an engine is operated at, thereby pumping more air through the engine. Unfortunately, high RPM operation requires cam lobe designs which are inefficient at low RPM, and is also stressful on engine parts.
An alternative method for increasing power production is to pump (or force) air into the engine. This approach is commonly called supercharging because more air is forced into each cylinder than 100 percent VE produces. For many years, supercharging was limited to special applications because of the power required to operate the supercharger (i.e., the parasitic draw of the supercharger) resulting in reduced fuel economy under all operating conditions.
One known supercharger is a screw compressor type supercharger employed to pump air into the engine at greater than atmospheric pressure to increasing horsepower. Screw compressor superchargers employ a pair of rotating screw elements (or rotors), within a confined cylindrical housing. The rotating screw elements draw air from a throttle body at a rear end of the housing and push the air progressing toward a forward end of the housing thereby compressing the air. The compressed air then flows into an intake manifold of the internal combustion engine. Providing the compressed air (commonly referred to as boost) and a corresponding amount of fuel, dramatically increases engine horsepower production and allows immediate and tremendous acceleration.
Twin screw type superchargers draw air into the rear of the supercharger and compress the air as it travels from the rear to the front of the supercharger between supercharger rotors, resulting in high pressures at the front of the supercharger. Because the rear of the supercharger must be open to provide a passage for air to enter the supercharger housing, the rotor (or timing) gears are generally at the front of the supercharger, along with rotor shaft bearings, and lubricating oil is present to lubricate the rotor gears and bearings. Front rotor shaft seals are necessary to prevent hot compressed air in the front of the housing from escaping from the housing and heating the lubrication oil, thereby reducing the effectiveness of the oil and causing gear and/or bearing failure, and to prevent the lubricating oil from leaking into the interior of the supercharger.
An unresolved weakness of twin screw superchargers has been the reliability of front rotor shaft seals at high boost levels. While the seals work well at between eight and twenty pounds of boost, increased wear has been observed above twenty pounds of boost. In the past, when boost was typically below twenty pounds the seal failure was not a significant problem. However, modern twin screw superchargers often produce greater than twenty pounds of boost and as a result, seal reliability has become a significant issue. Further, during part boost or no boost, the front rotor shaft seals are known to fail under vacuum and allow the lubricating oil to enter the supercharger interior.